Cells mount responses to extracellular stimuli by triggering changes in levels of second messengers, such as calcium, which then activate intracellular mediators of physiological response, such as calmodulin. Understanding how calmodulin-related proteins function will be valuable for elucidating signal transduction mechanisms in organisms.The purpose of this research is to determine the physiological and biochemical functions of the Arabidopsis calmodulin-related protein, TCH2. TCH2 mRNA accumulates in response to touch, wind, wounding, and darkness (Braam and Davis, 1990). TCH2 protein is 44% identical to calmodulin (Khan et al., 1997), suggesting a role for Ca2+ in TCH2 function. It is proposed that TCH2 is activated by Ca2+ and is involved in regulating a signaling pathway that translates external stimuli into a physiological response. To test this hypothesis, TCH2 mutants will be sought and characterized to determine the physiological function of TCH2. The biochemical role of TCH2 will be explored by screening for genes that encode proteins that bind TCH2 in a Ca2+-dependent manner. This research will provide insight as to how cells use Ca2+ to sense and respond to extracellular stimuli.